It is known to prepare dihydric phenols by condensing carbonyl compounds, such as aldehydes and ketones, with aromatic hydroxy compounds such as monohydric phenols under the influence of alkaline or acidic condensation catalysts. However, this reaction, particularly when an acid catalyst is employed, suffers from the disadvantages of relatively low yields of the dihydric phenol and the formation of undesirable byproducts. Thus, for example, the reaction of phenol with formaldehyde in the presence of dilute mineral acids produces not only 4,4'-dihydroxydiphenyl methane but also a number of other isomers, particularly the 2,2'-dihydroxydiphenyl methane and the 2,4'-dihydroxydiphenyl methane, as well as resinous polycondensation products of the Novolak type. The formation of these resinous by-products can be retarded by either using a large excess of phenol or carrying out the reaction in a homogeneous solution in dilute mineral acid. However, even at best the yield of 4,4'-dihydroxydiphenyl methane has been reported by workers in the art as being only about 35%, and that of all isomers as being 70%, calculated on phenol.
There thus exists a need for a process for the preparation of dihydric phenols, particularly unsymmetrical dihydric phenols, which are substantially free of undesirable byproducts, such as resinous byproducts, and which yields the desired dihydric phenols in relatively large amounts. It is an object of the instant invention to provide such a process.